The present invention generally relates to radio frequency power transistors, and more particularly relates to the detection of the onset of avalanche breakdown in a radio frequency power transistor to prevent damage of the device.
Wireless devices such as cellular phones, personal digital assistants (PDAs), laptop computers, and wireless controllers are being embraced by the consumer because they eliminate the need for wired connections. Moreover, wireless interconnectivity can be achieved with little degradation when compared to wired networks. Each wireless device transmits or receives data. The frequency at which these devices operate typically exceed 500 megahertz with many operating in a frequency range of 1.5 gigahertz to 5 gigahertz.
A radio frequency (RF) amplifier is incorporated in a wireless device to transmit the information. In general, the radio frequency amplifier is connected to an antenna to disperse the signal in a directional or non-directional pattern depending on the application. The power output of the radio frequency amplifier can vary greatly. Hand held devices such as cellular phone may require less than a quarter watt output to transmit to the nearest cellular receiver. Conversely, a television station may require amplifiers having kilowatts of power.
The power transistor(s) used in a RF amplifier typically operate in a gain configuration where the device is non-saturated. In general, the load on the RF amplifier is an antenna. The impedance of the load is matched to the impedance of the RF amplifier to efficiently transmit power. Voltage standing wave ratio (VSWR) is a measure of how close the load impedance and amplifier impedance match. Failure to match impedances will produce standing waves that result in signal reflection at the load. Ideally, the impedance of the load stays fixed. In practical applications this is far from the truth. For example, a cellular phone has a RF amplifier and an antenna that under normal operating conditions can be placed in a limitless number of environments. The impedance of a load such as an antenna will vary with motion and position thereby creating a condition where a load mismatch occurs. In some cases, a large voltage transient is produced that can damage the power transistor, in part, because the loading is inductive. Catastrophic damage is possible when the output voltage exceeds the breakdown voltage of the RF power transistor producing a situation (avalanche breakdown) where the safe operating area of the device is exceeded.
One prior art methodology uses passive components to protect the RF amplifier by limiting the voltage that is coupled back to the amplifier. Passive components such as resistors, capacitors, and inductors are used to form the protective circuit. Although they can be used successfully to reduce the threat of damage, the passive components also introduce undesirable effects on amplifier performance. For example, power gain, amplifier efficiency, amplifier linearity, and manufacturing cost are all parameters that suffer when adding these discrete components to enhance amplifier protection.
Thus, it should be appreciated that it would be desirable to provide a RF power transistor that could operate close to the limits of voltage breakdown to increase the efficiency of operation. In addition, it is desirable for the RF power transistor to provide a signal indicated the onset of avalanche breakdown to prevent this damaging condition from occurring. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.
A radio frequency power transistor is provided. The radio frequency power transistor has a collector, a base, an emitter, a first output, and a second output. The radio frequency power transistor comprises a first transistor cell having a base ballast resistor and a second transistor cell having an emitter ballast resistor. The operating differences of the first and second transistors cells are monitored. The first and second transistor cells generate a first difference voltage when the radio frequency power transistor is operating at voltages below where avalanche breakdown occurs. The first and second transistor cells generate a second difference voltage at the onset of avalanche breakdown.